I've always found the use of my retail skills to work with situations like these. The key is to focus on the better, without hitting too hard on the fault itself. You could call it avoiding the subject, but with sometimes, less is more when you're in that kind of position. One of the best ways to start this is so start with something like "Why don't we try…." as this leaves the understanding that you're experimenting and trying something new, while still maintaining control of the desired outcome. When the chemistry is right with both sides of the couple, things should start to spark as primal nature takes over when the desired target(s) are reached.
It's a bit of a different way of thinking of a solution, but it's a method that works great for a number of things, time and time again. It accomplishes the desired result (in this case, fixing a bad kisser), while putting the focus on the desired end result, without the need to point out what could be seen as a bad or negative aspect, or mentioning anything that would have a potential to become a sensitive subject matter. It's a rather different manner, but sometimes, it's the most effective.
